Optical modulating and amplifying apparatus capable of achieving modulation with high signal-to-noise ratio

Abstract: An optical modulating and amplifying apparatus includes an optical amplifier 3 and a modulator 4. In the apparatus, an input optical signal S1 and an excited light SP are combined with each other by an optical mixer 31 to produce an output light signal. The excited light SP from a laser diode 40 is sensed by a photodiode 41. A bias and an amplitude of the sense signal are respectively sensed by a means value sensor 51 and a modulation signal sensor 52. A modulation degree of the excited light SP is computed by a divider 54. A control signal to develop a predetermined modulation degree is delivered from a control signal generator 45 to a variable gain amplifier 43. In response to the control signal, the amplifier 43 controls the amplitude of the oscillation signal from an oscillator 44. A laser diode driver 42 adds a bias to the oscillation signal to drive the laser diode 40. The modulation degree is kept unchanged regardless of variation in an output level of the excited light so as to achieve the modulation with a high signal-to-noise ratio.

Description: The present invention relates to an optical modulating and amplifying apparatus, and in particular, to an optical modulating and amplifying apparatus in which a modulation signal is superimposed onto a signal light. Heretofore, a conventional optical modulating and amplifying apparatus includes. as shown in Fig. 1. an optical amplifier 1 including an optical mixer 11 for mixing an input light signal S1 to undergo modulation with exciting light SP undergone modulation and an optical fiber 12 using erbium. In addition to the optical mixer 11 comprising the optical amplifier 1, there is disposed a modulator 2 for supplying the exciting light SP to the mixer 11. The modulator 2 includes a laser diode 20 for producing exciting light. a laser diode driving circuit 21 for supplying a bias signal to the laser diode 20, and an oscillator 22 for producing and delivering a modulation signal to the driver 21. In the optical amplifier modulator above, the modulation signal from the oscillator 22 and the bias signal from the driver 21 are used to create a driver current. The current drives the laser diode 20 to emit exciting light SP. The modulated light SP is then fed to the mixer 11 to be superimposed onto the input optical signal S1. The resultant signal is propagated through the fiber 12 to be outputted therefrom as an optical signal S2. Namely, the modulated signal SP is used to alter the gain of the optical amplifier 1 for the modulation of the input optical signal S1. However, in the conventional apparatus, since the modulated output signal delivered from the oscillator 22 has substantially a fixed output level. variation in the output level of the exciting light SP from the diode 20 causes the modulation degree to be also changed. In a case where the degree of modulation is increased. the modulation signal becomes noise of the signal undergone the superimposition and hence the signal-to-noise (S/N) ratio of the signal is deteriorated. That is, the contents of the input light signal S1 is influenced from the exciting signal SP. On the other hand, when the modulation degree is decreased, there appears another problem that the signal superimposed onto the input signal S1 cannot be easily sensed. Referring now to the drawings, the problem above will be further described. As can be seen from Fig. 2A, the exciting light SP1 is a signal having a direct-current (dc) level P1. Using the dc level P1 as a bias, a modulation signal fm (with an amplitude P0) from the oscillator 22 is superimposed onto the exciting light SP1 so as to obtain an exciting light SP2. Namely, the modulation degree ml is P0/P1 in this case. However, as shown in Fig. 2B. when the dc level of the exciting light SP1a is altered to P2 with the amplitude P0 kept unchanged in the signal fm, the modulation degree m2 of the modulated light SP2a is P0/P2. As above, the modulated light SP2a has the amplitude P0 which is larger than the dc level P2. In consequence. the modulation signal fm appears as noise for the exciting light SP1a and adversely influences the gain related to the input signal S1. Namely, the signal-to-noise ratio is deteriorated for the output signal S2. It is therefore an object of the present invention to provide an optical modulating and amplifying apparatus capable of achieving a modulation with a high signal-to-noise ratio. To achieve the object above, there is provided in accordance with the present invention as defined in claim 1, an optical modulating and amplifying apparatus in which according to an oscillation signal produced from an oscillation means, laser light driving means drives laser light emitting means to emit an exciting light such that an input signal thereto is optically amplified according to the exciting light. The apparatus includes control signal generating means for sensing the exciting light and producing a control signal to set a modulation degree of the excited light to a predetermined modulation degree, and modulation signal control means for receiving as inputs thereto the control signal and the oscillation signal to produce a modulation signal such that the laser light driving means drives the laser light emitting means according to the modulation signal. In addition. the apparatus is favorably configured as follows. The laser light driving means drives the laser light emitting means according to a signal obtained by adding a dc-level bias to the oscillation signal. The control signal generating means includes mean value sensing means. a modulation signal sensing means, and an operating means. The mean value sensing means senses a dc level of the exciting light, the modulation signal sensing means detects an amplitude of a modulation signal superimposed onto the exciting light, and the operating means computes the modulation degree according to the dc level and the amplitude of the modulation signal. The modulation signal control means increases or decreases the amplitude of the oscillation signal according to the control signal produced from the control signal generating means. The optical modulating and amplifying apparatus further includes laser light sensing means for sensing the exiting light emitted from the laser light emitting means. Consequently, in accordance with the optical modulating and amplifying apparatus of the present invention. the control signal generating means creates a control signal to set the modulation degree of the exciting light thus sensed to a predetermined value, the modulation signal control means produces a modulation signal based on the control signal and an oscillation signal, and the laser light driver means drives the laser light emitting means depending on the modulation signal. Namely, there is formed a loop circuit configuration which controls the operation to drive the laser light emitting means emitting the exciting light. The objects and features of the present invention will become more apparent from the consideration of the following detailed description, taken in conjunction with the accompanying drawings. wherein: Fig. 1 is a schematic block diagram showing an example of the basic configuration of the conventional optical modulating and amplifying apparatus; Fig. 2A is a graph showing signal waveforms with the modulation degree set to m1 for explaining a relationship between the amplitude of the modulation signal and the modulation degree in the apparatus of Fig. 1; Fig. 2B is a graph showing signal waveforms with the modulation degree set to m2 useful to explain a relationship between the amplitude of the modulation signal and the modulation degree in the apparatus of Fig. 1; Fig. 3 is a block diagram showing an optical modulating and amplifying apparatus in an embodiment in accordance with the present invention; Figs. 4A to 4C are graphs showing signal waveforms for explaining sensing operations in a control signal generator; and Fig. 5 is a graph showing signal waveforms useful to explain a relationship between the amplitude of the modulation signal and the modulation degree in the apparatus in accordance with the present invention. Referring now to the accompanying drawings, description will be given of an embodiment of the optical modulating and amplifying apparatus in accordance with the present invention. Figs. 3 to 5 show representative aspects of the embodiment of the apparatus capable of conducting the modulation with a high signal-to-noise ratio. Fig. 3 shows in a block diagram the circuit constitution of the apparatus, whereas Figs. 4A to 4C and Fig. 5 are explanatory diagrams of the control operation of the embodiment. In Fig. 3. the apparatus of the embodiment includes an optical amplifier 3 and a modulator 4. The amplifier 3 includes an optical mixer 31 for combining an input optical signal S1 to undergo modulation with exciting light modulated and an optical fiber 32 using erbium. The amplifier 3 produces an output light signal S2. The modulator 4 includes a laser diode (to be abbreviated as LD herebelow) 40 as laser light emitting means for emitting exciting light SP, a photodiode 41 as laser light sensing means for converting the exciting light SP from the diode 40 into an electric signal. a laser diode driver circuit 42 as laser light driving means to feed a dc level bias signal to the laser diode 40. a variable gain amplifier 43 as modulation signal control means for delivering a modulation signal to the driver 42, and an oscillator 44 as oscillation means to produce a source signal of the modulation signal. There is further included a control signal generator 45 as control signal generating means for supplying a control signal to the amplifier 43. The generator 45 includes a mean value sensor 51 as mean value sensing means to receive a sense signal from the photodiode 41 so as to sense a means value of the exciting light SP from the laser diode 40, a modulation signal sensor 52 as modulating signal sensing means for receiving a sense signal from the photodiode 41 and sensing an amplitude of the modulation signal component of the emission signal from the laser diode 40. an analog-to-digital (A/D) converter 53 for converting each of the analog sense signals respectively from the sensor circuits 51 and 52. and a divider circuit 54 as operating means for effecting a division between the digital signals from the A/D converter 53. In the modulator 4 constructed as above, according to the exciting light SP from the laser diode 40, there is formed a loop circuit including the photodiode 41, the control signal generator 45, the variable gain amplifier 43, and the laser diode driver 42. In the loop circuit, according to the oscillation signal from the oscillator 44, the gain of the amplifier 43 is controlled by the generator 45, thereby regulating the stable emission of the laser diode 40. The means value sensor 51 of the control signal generator 45 is implemented, for example, as a smoothing circuit including an integrating circuit. Moreover, the modulation signal sensor 52 is configured, for example, with a dc breaking capacitor, not shown. Fig. 4A shows in a graph a signal characteristic of the exciting light SP sensed by the photodiode 41, whereas Figs. 4B and 4C respectively show a direct-current (dc) component and an alternating-current (ac) component of the signal. The mean value sensor 51 senses a dc level Px of the exciting light according to the sense signal from the photodiode 41 as shown in Fig. 4B. The modulation signal sensor 52 extracts an ac component of the signal as shown in Fig. 4C so as to obtain an amplitude Py of the modulation signal according to the peak and bottom values of the extracted ac component. In addition, the divider circuit 54 conducts an operation Py/Px to attain the modulation degree according to the dc level Px from the sensor 51 and the ac level Py representing the amplitude of the ac component from the sensor 52. Furthermore, the modulation degree is compared with a preset modulation degree having a predetermined level. Based on the result of the comparison and the dc level Px from the sensor 51, there is computed an amplitude value as a modulation degree at the predetermined level so as to produce a control signal according to the obtained amplitude. The control signal is then fed to the variable gain amplifier 43. The amplifier 43 is responsive to the control signal from the generator 45 to control the amplification degree so as to set the amplifier of the oscillation signal from the oscillator to a favorable amplitude value associated with the modulation signal, thereby delivering the amplified modulation signal to the laser diode driver 42. The driver 42 receives the modulation signal from the amplifier 43 and then adds a predetermined bias signal thereto to produce a resultant signal. In response to the signal from the driver 42. the laser diode 40 is driven to emit laser light therefrom. The exciting light SP produced from the laser diode 40 is fed to the optical mixer 31 of the amplifier 3. In response to the light SP, the amplifier 3 alters the gain thereof to modulate the input optical signal S1. In the amplifier 3, the signal S1 is amplified according to intensity of the light SP from the diode 40 so as to resultantly create an output optical signal S2. Description will now be given of the amplification of the amplifier 3, particularly, a relationship between the modulation degree and the amplitude of the output signal S2. As can be seen from Fig. 2A, the degree of modulation m1 (m1 = P0/P1) in which the modulation signal fm having the amplitude P0 is superimposed onto the output level P1 of the exciting light SP is assumed to be the predetermined modulation degree above. Under this condition. assume the dc level of the exciting light SP is decreased from P1 to P2 as shown in Figs. 2A and 2B. The value of the dc level of the exciting light SP is sensed by the mean value sensor 51, whereas the value of the amplitude of the modulation signal fm is measured by the modulation signal sensor 52. The obtained values are respectively converted through the A/D converter 53 into digital values to be transmitted to the divider 54. On receivng the dc level value and the amplitude value, the dividing circuit 54 computes a modulation degree m2 in a realtime fashion as shown in Fig. 5. Comparing the attained modulation degree m2 with the preset modulation degree ml. there is obtained an amplitude value currently suitable for the dc level of the exciting light SP so as to produce a control signal associated with the computation result. Based on the resultant control signal, the gain of the variable gain amplifier 43 is controlled for the oscillation signal from the oscllator 44. The amplitude of the oscillation signal is thus regulated through the gain control of the amplifier 43 to have an appropriate amplitude P0a, thereby feeding the signal to the laser diode driver 42. In the driver 42, an appropriate bias is applied to the oscillation signal so as to thereby drive the laser diode 40. In response thereto, the laser diode 40 creates exciting light SP having the modulation degree ml. The control signal generator 45 sequentially achieves the control operation of the exciting light SP. In consequence, even when the output level of the excited light is altered, the degree of modulation can be kept unchanged. This suppresses the deterioration of the signal-to-noise ratio of the transmission signal and facilitates detection of the superimposed signal. In this connection, the embodiment has been described only as an example of a suitable implementation of the present invention. The present invention is not restricted by the embodiment and can be changed or modified without departing from the scope of the invention. For example, the control signal generating circuit 45 is not limited to the configuration employing the mean value sensor 51, the modulation signal sensor 52, the A/D converter 53, and the dividing circuit 54 but can be configured in various circuit structures. Moreover, the variable gain amplifier 43 may be constructed with an attenuator. As above, in the optical modulating and amplifying apparatus in accordance with the present invention, the control signal generating means, the modulation signal control means. the laser light driving means, and the laser light emitting means constitute a loop circuit achieving a control operation to set the degree of modulation of the excited light to a predetermined value. In consequence, since the predetermined modulation degree of the excited signal is kept unchanged through the control operation, there is attained a satisfactory signal-to-noise ratio for the modulated signal to be transmitted; moreover, the modulation and amplification of the input optical signal are efficiently conducted.

Claims: An optical modulating and amplifying apparatus having oscillating means (44), laser light driving means (42), and a laser light emitting means (40) in which the laser light driving means (42) drives according to an oscillation signal generated from the oscillating means the laser light emitting means to emit exciting light into an optical amplifier (3), characterized by further including control signal generating means (45) and modulation signal control means (43), wherein the control signal generating means (45) senses the exciting light outputted from the laser light emitting means (40) and produces a control signal to set a modulation degree of the exciting light to a predetermined modulation degree; the modulation signal control means (43) receives as inputs thereto the control signal and the oscillation signal generated from the oscillating means (44) and produces a modulation signal, and the laser light driving means (42) drives according to the modulation signal the laser light emitting means (40) to emit the exciting light into the optical amplifier (3), thereby achieving an optical amplification of an input signal according to the exciting light. An apparatus as claimed in claim 1. characterized in that the laser light driving means (42) drives the laser light emitting means (40) according to a signal obtained by adding a dc-level bias to the oscillation signal. An apparatus as claimed in claim 1 or 2, characterized in that the control signal generating means (45) includes mean value sensing means (51), a modulation signal sensing means (52), and an operating means (54), the mean value sensing means (51) sensing a dc level of the exciting light, the modulation signal sensing means (52) sensing an amplitude of a modulation signal superimposed onto the exciting light, and the operating means (54) computing the modulation degree according to the dc level and the amplitude of the modulation signal. An apparatus as claimed in claim 1, 2 or 3, characterized in that the modulation signal control means (43) increases or decreases the amplitude of the oscillation signal according to the control signal produced from the control signal generating means (45). An apparatus as claimed in any one of claims 1 to 4, wherein the optical modulating and amplifying apparatus further includes laser light sensing means (41), the laser light sensing means (41) sensing the exciting light emitted from the laser light emitting means (40).

Inventors: SAKUYAMA HIROSHI, SAKUYAMA, HIROSHI, SAKUYAMA, HIROSHI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1994
Main classification: H01S3
IPC codes: H01S5, H01S3, H04B10